In general, a pneumatic tire comprises a carcass ply reinforced by a number of carcass cords extending between bead portions and turned up around bead cores from the inside to the outside of the tire so that the carcass ply is secured to the bead cores.
In recent years, on the other hand, in view of environment improvements, tire manufactures make great efforts to reduce the weight of passenger car tires as well as heavy duty pneumatic tires.
By decreasing the size of the carcass ply turnup portions, the tire weight can be reduced.
However, in the case of the passenger car tires, there is a trend toward very low tire aspect ratios. In other words, the radial dimension of the tire sidewall portions becomes very small. Therefore, it is difficult to reduce the size of the carcass ply turnup portions any more.
In the case of the heavy duty pneumatic tires, the tires have a high probability of being used under extremely severe service conditions with heavy loads and very high tire inflation pressure, therefore, if the size of the carcass ply turnup portions is decreased, there is a possibility that the carcass ply turnup portion is slipped out of the bead core.
Further, as a result of tire weight saving, the bead portions and sidewall portions of such a tire are more deflected or bent during running, therefore, the carcass ply turnup ends are more likely to undergo larger bending/compressive stress, and separation failures from the surrounding rubber tends to occur starting from the turnup ends.